EIS Gambling With Grout 28012016

Offshore Wind Structures: Gambling With Grout – Worth The Risk?
The Engineering Integrity Society - 4th Durability & Fatigue Advances in Wind,
Wave and Tidal Energy BAWA, Filton, 28th January 2016
Dr. C. R. Golightly GO-ELS Ltd. – Offshore Wind Structures: Gambling With Grout – Worth The Risk?
EIS 4th Durability & Fatigue Advances in Wind, Wave and Tidal Energy BAWA, Filton, 28th January 2016
Dr. Chris Golightly GO-ELS Ltd.
Geotechnical & Engineering Geology Consultant

Presentation Contents
Differences; Oil & Gas Platforms – Wind Turbines
Foundation Concepts 2012 – 2020 [Berger 2013]
Types of Turbine Foundation for OWT
History of Monopile [MP] Connections
Advantages and Disadvantages
Applications & Factors Affecting Ultimate Strength (Refs. 1 & 2)
DNV OS J101 Design Code & Shear Keys (Refs. 5 to 7)
Offshore Wind - Brittle High Strength Grouts
Monopile Vs Tripod/Jacket Loading
RWE Gwent Y Mor Grouting Study (Ref. 8)
JIP Conical Connections
Trelleborg Elastomeric Spring Bearings (Refs. 3, 4 & 11)
Pile Swaging & Slip Joints (Ref. 19)
Integral and External Mating
Quick Coupling; Integral MP & TP
Fatigue Life; OWI-LAB BELWIND (Refs. 3 & 4)
Conclusions, References, Contact Details

Differences; Oil & Gas Platforms – Wind Turbines
Oil & Gas Platforms
 Relatively stiff structures, usually
founded on long driven piles and
mudmats
 Axial loads dominate due to high
structure weights
 Structural dynamics are not critical with
weight >>> bending moments
 Wave loads tend to dominate design in
high energy areas such as North Sea
 Straightforward Force – Response
relationship
 Each design is one-off “Prototype” at a
single location
Offshore Wind Turbines
 Relatively flexible towers on variety of
foundation types, monopiles 4 to 9 m
diameter, tripods/4 leg jackets, GBS.
 Structural dynamics always critical. 3P
Eigenvalue resonance
 Bending moment and lateral response
more important than axial load
 Wind and wave loads both very
important
 Complex uncorrelated/uncoupled
loading
 Large Nos. of OWT in arrays (80 [German AV
Tripods] to 2000 [FOREWIND Statoil UK])

Foundation Concepts 2012 – 2020 [Roland Berger Study 2013]
Offshore Wind Foundation – Definition. The “Sub-Structure”

Types of Foundation for Offshore Wind Turbines [OWT]
Choice of foundation solution influenced by:
• Water depth and seabed conditions,
especially depth to rockhead.
• Environmental loading (wind, wave, tidal).
• Onshore fabrication, storage and
transportation requirements.
• Offshore vessel & equipment spread costs
& availability.
• Installation & Construction methodology
available.
• Developer CAPEX investment appetite and
OPEX (Repair & Maintenance) predictions.
Smarter solutions available (suction caissons,
GBS, lighter jackets/trusses, hybrids, seabed
anchored templates).
“Foundations” (or sub-structure) 30 to 40%
of CAPEX & rising. Cost reductions essential.
“Smarter” lighter hybrid foundations needed
& move away from riskier costly conventional
driven tubular steel piling.
Source: UPWIND Project Final Report 2011
Source: NREL

History of Monopile Connections
 1st Offshore Wind UK Round 1 Blyth and Scroby Sands MP projects in 2001 used bolted
pre-fitted welded flange connections. This technique was rejected in favour of a
cheaper, quicker “more efficient” grouting technique.
 At the end of 2009 grouted connection joints, between large diameter monopiles [MP]
and connecting tubular steel transition pieces [TP] at the base of overlying support
towers, were found to be failing by cracking and slipping.
 For the majority of 70% of UK offshore MPs which experienced grout cracking,
settlements and failures, this was primarily due to widespread absence of shear keys (or
weld beads) on the surfaces of straight “plain pipe” MP and transition piece TPs.
 Bending moments as a result of complex wind and wave loading are an important
design consideration, which had been underestimated in design.
 Axial connection capacity was found to be very significantly lower than assumed due to
MP scale effect, lack of manufacturing and installation tolerances and abrasive wear due
to the sliding of contact surfaces subjected to large moments.
 Typical failure modes for the brittle rock-like grout [cement] connections include
disbonding, cracking, wear and compressive grout crushing failure.
 These failures resulting from a systemic design fault have necessitated assessment and
repairs which have not all been fully reported publically and there have been a number
of claims and arbitration cases.

History of Monopile Connections

Advantages and Disadvantages
ADVANTAGES
 Liquid grout displaces sea-water from annulus, allowing for construction imperfections
- Out of roundness and straightness
- Tilt and Offset (variable annulus)
- Surface irregularities (plate rolling imperfections, welds, surface condition etc)
 Easy verticality alignment
 Attachment of secondary steel directly onto TP
 Construction methods well established and fast
 Competitive construction market and costs
DISADVANTAGES
 Relies on durability of the grout seal
 Backing plates – double seals
 Inspection is difficult or indirect
 Certification procedure
 Curing time before installation of tower
 Expensive shear keys

Applications & Factors Affecting Ultimate Strength (Refs. 1 and 2)
APPLICATIONS
 Pile / sleeve or pile / leg connections in multi-legged jacket structures
 Structural connections between sections of deep water jackets
 Sub-sea strengthening & repairs
 Offshore Wind Monopile / TP and Tripod/Truss Tower connections
FACTORS AFFECTING ULTIMATE STRENGTH
 Cross sectional geometry
K = [(Dp/tp) + (Ds/ts)]-1 + (1/m)(Dg/tg)-1
 Grout uniaxial compressive strength fcu
 Shear key height h & spacing s (h/s)
 Size or scale Cp
 Length : diameter L/Dp
 Surface condition
 Manufacturing Imperfections
 Early age cycling
Ds
Dg
Dp
tp
tg
ts
s
L
1
23
h
4

DNV OS J101 Offshore Wind Turbine Design Code and Shear Keys
 The 2 UK offshore wind MP projects with no failures included shear keys, common
practice for oil and gas (API RP2A). Designers had Oil & Gas industry experience.
 DNV J101 (2007) offshore wind turbine design code left it open to designers
whether to use shear keys/weld beads or not.
 Many designers did not include shear keys as this was perceived to be a cheaper,
quicker option and thought shear keys led to “stress concentrations”.
 MP-TP annulus grouting allowed easier, quicker adjustment of the pile out-of-
verticality using jacking to level the turbine tower prior to grouting.
 This was essentially a systemic design error as a result of code phrasing omissions.
Many projects are now adopting direct bolted flange connections or unified MP-TP.
 Use of straight “Plain Pipe” non shear keyed connections not recommended and is
now discontinued.
 Some MP projects now adopt 1-3 deg. conical designs without shear keys, presumed
to “catch” the TP as the connection settles and drops, allowing radial stresses to be
regained. This might be regarded by some as “engineering for failure”.
 Industry best practice and DNV code guidelines now extensively reviewed, and
revised in 2011 (latest Code 2014, Refs 5, 6 & 7). There are still some anomalies in
behaviour. Research is ongoing on scale & fatigue effects but situation clearer.

Offshore Wind Industry Adoption of Brittle High Strength Grouts
 Tried and tested appropriate underwater grouts were originally used to cement piles
into bedrock, amongst other applications. This technique was then adopted over 12
years ago for offshore wind turbines, as a more efficient alternative to bolted
flanges, which assisted in levelling towers to vertical.
 Typically, brittle high and ultra high strength grouts used have UC strengths > 100
MPa up to 200 MPa. In a geological context, this is a “Very Strong” rock which could
“only be chipped by heavy hammer blows”, according to standard rock engineering
strength descriptions. They exhibit high ratios of compressive to tensile strength.
 It is not difficult to envisage twin large diameter steel tubes sandwiching an annulus
of such “rock” cracking & crushing, leading to progressive failure at the top and base
as piles are cyclically loaded by wind and waves over long periods. Patterns of
cracking measured are reputedly linked to predominant environmental load
directions.
 The MP grout failures may have been related to manufacturing, installation and
positioning tolerance uncertainties and out-of-roundness which in some cases have
led to MPs and TPs both being slightly out of shape, with the grouted annulus
thicknesses therefore varying vertically. Little to nothing is published on this.
 There have been question marks over the long term fatigue strength of HPC grouts,
following work by Anders & Lohaus (2007) and Soerensen et al (2011).
 There are suggestions in the work done now that a lower strength less brittle grout
may be more appropriate for use in some designs, should grout be adopted. There
is a need to "bottom-out" the potential water ingress and cyclic fatigue problems.

 “Monopiles” with D/t ratios often in excess of 100 are in reality “thin-walled steel
caissons” rather than piles.
 MP ability to transfer large moments is complex, but has become better understood.
Design theories still have limitations & shortfalls. The use of conical TP sections
[“controlled engineering for failure”] is uncertain in the long term.
 High dead weight oil & gas platforms have used API RP2A designed grouted leg-pile
connections for decades, but stresses are usually predominantly compressive. However
OWTs are low deadweight loaded, highly cyclic, with complex vertical & bending force
coupling, with tensile stress zones in the grout.
 Dynamic load regimes experienced by the legs of tripods (Germany) and 4-leg jackets
(mostly UK) are different to the predominant bending mode experienced by MPs.
 Some tripod and jacket designs include “stopper plates”. (e.g. Borkum West 2) These
“belt-and-braces” designs suggest a lack of confidence in the robustness under long term
cyclic fatigue conditions over a 20+ year design life.
 It is uncertain whether or not tripods/jacket grouted connections will experience fatigue
degradation in time, even with the provision of shear keys. There has been extensive
research especially at Leibnitz University Hannover (Refs. 10, 16, 17 & 18).

For smaller tripod/jacket piles of diameter up to ~
2.5 m, work mostly in Germany led by German
BSH committee has illustrated a different "push-
pull" loading regime to monopiles. No theoretical
reason why grout should not be used with shear
keys, correctly designed and installed/constructed.
Two methods used for German AV tripods &
jackets:
1. Tripods lowered onto template pre-driven pile
groups [e.g. Borkum West 2] or:
2. Piles vibrated then driven through sleeves of
pre-placed tripods [e.g. Global Tech 1].
Option (2) preferable, since tripod or jacket leg
sits inside the pile. Outside requires more
complicated sealing and has different grout stress
pattern.
Similar to Ormonde Irish Sea project, where jacket
legs stabbed inside pre-installed seabed pile
template, with wide annulus of lower strength
grout to allow for installation tolerances. Current
industry favoured solution.

RWE Gwent-Y-Mor Study 2011
 Julian Garnsey of RWE led a study published in 2011 assessing grouted connections
for the GYM monopiles (Ref. 8).
 The project assessed eight generic concepts:
1) Grouted conical without shear keys; 2) Grouted cylinder with shear keys; 3) Bolted flange;
4) Bracket support; 5) Swaged Connection; 6) Integrated MP and TP; 7) Pinned Connection;
8) Clamped Connection
 The bolted flange proved the most promising for further detailed investigation, but
was not selected because the change would have impacted on the project schedule.
 Five concept variants emerged from this process as potential solutions;
1) Grouted conical without shear keys and an axial bearing system
2) Grouted cylinder with shear keys and an axial bearing system
3) Bolted flange internal to the transition piece above high tide level
4) Conical grouted connection without shear keys and an axial bearing system
5) Conical grouted connection without shear keys.
 Concluded best solution is shear keys in the middle third and a longer connection
plus necessary “back-up” elastomeric bearing support, allowing lower strength grout.
This more robust connection with reduced contact pressures at the ends and a less
brittle grout would reduce cracking around the shear keys and connection ends.
Necessary to confirm zero water ingress does not adversely affecting the grout
matrix, which would lead to a simplified seal specification.

RWE Gwent-Y-Mor Study 2011
The methodology developed resulted in a numerical weighting for EIGHT criteria as
follows, with the subsequent ranking shown:

JIP Conical Connection Solution 2011
 A Joint Industry Project [JIP] was carried out by DNV to investigate the structural
capacity of these connections from autumn 2009 to January 2011 (Refs 5, 6 & 7).
 Axial capacity found to be more sensitive to diameter and surface/positioning
tolerances than allowed for in existing design standards
 Design procedure with conical shaped connections was developed.
 January 2011 JIP on capacity of cylindrical shaped grouted connections with shear
keys initiated. Analytical design equations developed for ULS and FLS.
 Recommended design methodology supported by laboratory tests.

Trelleborg Spring Bearings Solution – MP to TP Support
 Several projects adopted or are adopting retro-fitted/new Trelleborg spring bearings (BELWIND
[330], Robin Rigg [360], Sheringham Shoal [540], Greater Gabbard 120], Rhyl Flats [158] and
Gwent Y Mor [960]). For a normal monopile foundation six bearings are required (see Ref. 11).
 Where elastomeric bearings were not fitted during construction and slippage occurred later, they may
be retrofitted by welding new brackets to the inside of the TP. The solution is used on installed wind
farms where grout problems occur but also as a precaution.
 At the start of construction, the bearings can be fitted so that they are unloaded and just resting at
the top of the MP. If slippage occurs at a later stage, the bearings are gradually loaded to assist the
grout in supporting the weight of the TP and tower assembly.
 A further function of the bearing is preloading to carry the static vertical load from the start with the
aim of preserving the strength of the grout.
 Long Term Measurement & Condition Monitoring is Essential

Pile Swaging and Slip Joints
 Many developers such as E.On for Amrumbank and Humber Gateway have reverted
to bolted flanges, with some considering integral MP & TP, pile swaging, quick
coupling lock rings, internal/external mating, or slip joints as reliable long term
solutions. All require control on verticality, careful driving. Costly.
 e.g. Beatrice project (DownVInD), where pile swaging (Hydra-Lok® system) used
for jackets, which secures structures by expanding piles radially into surrounding
sleeve in substructure. Easily monitored and quicker, but more expensive.

Integral and External Mating ?

Quick Coupling; Integral MP & TP ?
http://www.pes.eu.com/assets/misc_de
c/vanoordpdf-384857351196.pdf
Van Oord @ Luchterduinen and Gemini
[NL]:
“Next year in July we will start with
piling the first foundations. Those are
quite innovative as it is a combined
monopile and transition piece. We are
piling on the flange on which the wind
turbine tower will be mounted. After
piling the secondary steel will be
attached to pre-mounted brackets. By
doing this we prevent a grouted
connection between the mono pile and
transition piece. At the same time this
concept is cheaper than a foundation in
two parts”.

Fatigue Life (Lohaus & Anders 2007; Soerensen et al, 2011)
Lohaus & Anders (Ref. 13)
“The fatigue strength of UHPC in high-cycle fatigue seems to be lower compared to normal
strength concrete. Regarding UHPC in grouted Joints nearly bilinear load-deflection curves for
specimens with shear keys in uniaxial compression are shown, which represent two different
load-bearing mechanisms and a very ductile failure”
Soerensen et al (Ref 19)
• BASF 140 MPa high performance grout, a cementitious binder material containing microsilica
and other added minerals. Prepared at ultra-low water/cement ratio using superplasticizing
admixture. Aggregate natural sand (0-4 mm). Strength after 28 days curing in water at 20o C.
• In air, fatigue life comparable to ordinary concrete, but in water grout exhibited drastically
shorter fatigue life at stress levels in excess of 60% of static compressive strength.
• In air, loading frequency (0.35 Hz, 5 Hz, and 10 Hz) had no influence on the fatigue strength,
but in water the fatigue capacity was much lower at 0.35 Hz than at either 5 Hz or 10 Hz.
Reduction in fatigue life in water was particularly severe at the lowest frequency 0.35 Hz.
• Reduced fatigue capacity postulated due to water trapped during cyclic loading, exerting
internal pore pressures high enough to cause progressive crack formation (“micro-wedging”?).
Effect more pronounced at low loading frequencies, with time available for water ingress and
subsequent pressure build-up during each load cycle. Research ongoing.
• Fatigue life reduction in water was not observed at the lowest stress level investigated (45% of
the static compressive strength).

Fatigue Life (Lohaus & Anders, Ref. 13; Soerensen et al, Ref. 19)

Main Conclusions: Offshore Wind Foundations
1. Initially the relatively new offshore industry perhaps understandably used conservative
monopile, piled tripod (Germany) & 4-leg jacket (UK) solutions. CAPEX investment still
limited compared to other energy industries.
2. European Offshore Wind Industry has developed alternative foundation solutions,
monopiles, piled tripods, BARD tripiles, 3 & 4-leg jackets, truss towers, concrete GBS,
twisted jackets, guyed & A-frame MPs, monopod or triple/quad suction caissons.
3. The OW industry must be more realistic about excessively conservative offshore turbine
lateral nacelle tilt criteria [0.5 deg.], based upon sound engineering analysis. This
impacts whole structure costs. Development of tilt-tolerant DD turbines can reduce costs.
4. For OW foundation costs to reduce [must be halved - US DoE], innovative solutions are
needed, selected/tailored to specific site conditions. Conservative risk averse attitudes in
a relatively new industry will change as experience is gained & investment increases.
5. Main Foundation Risks: Grouted connections, piling noise mitigation, over-conservative
long, stiff, heavy pile design, pile tip buckling, internal & external corrosion, scour
protection and J-tubes, unplanned drilling/re-driving, tilt and settlement.
6. The industry current push [Project PISA] to move to ~10 m dia., 1200 Tonne, 60 m +
length monopiles in ~40 m WD may be questionable & should be challenged.

Main Conclusions: MP-TP Grouted Connections
1. Discovery from late 2009 onwards that > 70% of UK Monopile connections failed, settled,
slipped and cracked. This was a systemic design error over a long period.
2. Bolted flange or other direct connections are possible. If MP grouting is adopted use of
shear keys & robust grout seals is essential with overlap lengths of at least 1.25 *D
required [Schaumann]. Studies by Centrica/RES [Race Bank] and RWE [Gwent-Y-Mor]
indicate that the cost differences between bolted flanges and grouting are in fact minimal.
3. Developers are now (2015) adopting:
 Non shear keyed conical [Anholt, London Array],
 Bolted flanges [Amrumbank, Humber Gateway],
 Retro-fitted bearings [Robin Rigg, Sheringham Shoal]
 Shear keyed with new elastomeric bearings [BELWIND].
 New single piece MP-TP [Achterduinen]
4. Reviews of retro-fitted elastomeric bearing installations show these should be robust, with
condition monitoring and displacement measurement required over the design lifetime.
5. It is questionable whether or not non shear keyed conical [1o-3o] sections and/or will
remain “fully robust” for fatigue design lifetimes of 20+ years?
6. Questions regarding long term fatigue behaviour under high loading with water ingress?
7. Measurement, Monitoring and Mitigation for offshore structures is essential for long term
design life O&M cost minimisation. The BELWIND project is state-of-the-art (Refs. 3 & 4)

Offshore Wind; Measurement, Monitoring, Mitigation (BELWIND)
Poster Paper EWEA 2014 Barcelona – Vrij Universiteit Brussel – De Sitter et al

The Future: Floating Wind – Huge Potential Offshore Wind Resource
Majority of OW developments have been in the Southern North Sea, a relatively flat shallow water
continental shelf, mainly dense sand, stiff glacial clayey soils & soft sediment filled paleo-valleys.
Not globally representative. Most coastal areas are steep, rocky, with thin (< 5 to 10 m) soil cover.
Piling is costly for fixed or floating structures. Soils insufficient for drag or suction caisson anchoring.
Source: Statoil Global Offshore Wind 2014

Comparison Oil Drilling Semi-Sub Vs Offshore Wind Floater

The Future: Offshore Floating Wind Leaders
HYWIND Statoil [NO] statoil.com/en/TechnologyInnovation
PELASTAR Glosten [US] pelastar.com
WINDFLOAT Principle Power [PO/US] www.principlepowerinc.com/products/windfloat.html
IDEOL IDEOL Partners [FR] ideol-offshore.com/en
WINFLO DCNS-Alstom [FR] fr.dcnsgroup.com/produit/eoliennes-flottantes/
INFLOW EDF-IFP-Nenuphar [FR] inflow-fp7.eu/floating-vertical-axis-wind-turbine/
GICON GICON-Fraunhofer [DE] gicon-sof.de/en/sof1.html
FUKUSHIMA Mitsubishi-Hitachi [JA] fukushima-forward.jp/english/
DEEPCWIND 30 diverse members [US] composites.umaine.edu/our-research/offshore-wind/deepcwind-consortium/
SANDIA Sandia Labs [US] energy.sandia.gov/energy/renewable-energy/wind-power/offshore-wind
Source: Myhr et al, 2014.

Floating Wind Platforms – Semi-Sub - Spar - TLP - Taut Buoy

Vertical Axis Wind Turbines [VAWT]– Pros and Cons
ADVANTAGES
o Omni-directional
- accepts wind from any direction
o Components mounted at sea level
- ease of service & maintenance
- lighter weight composite structures
o Can theoretically use less materials to
capture the same amount of wind
DISADVANTAGES
o Rotors lower at reduced wind speeds
o Centrifugal force over-stresses blades
o Poor self-starting capabilities
o Often requires support at turbine
rotor top
o Rotor needs removing for bearings
replacement
o To date, poorer performance &
reliability than HAWTs

Closing Thoughts – Future of Offshore Wind Energy
Aim: Most Efficient Abstraction of Kinetic Energy From Moving
Turbulent Air [OFFSHORE WIND]
 How Would That Be Done in 2015 From A Standing Start? Fixed
Structure Top Heavy 3 Bladed Onshore HAWT on Fixed Steel
Towers?
>> No. Too Expensive and Subsidy Dependent
 What Will The Global Mix Be Between Fixed Vs Floating?
>> Deeper Waters/Sloping Seabeds >> FLOATING VAWT
 Will There be a Real Offshore Wind “Gamechanger” - or not? Yes
there must be soon.
>> [$$$ ECONOMICS $$$]

References (1)
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www.ice.org.uk/topics/energy/Recorded-lectures
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http://wiki-cleantech.com/wind-energy/the-future-of-monopile-grouted-connections-a-clients-perspective

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19. Van der Temple, J. (2011), “Slip Joint, Solving The Grout Problem” Proc. EWEA 2011, Amsterdam November
2011.
http://proceedings.ewea.org/offshore2011/programme/info2.php?id2=566&id=104%20&ordre=1
20. “Monopile Retrofits and Designs Going Forward: Room for Grout?”, April 11th 2011.
http://social.windenergyupdate.com/offshore/monopile-retrofits-and-designs-going-forward-room-grout
21. Wind Energy Update (2012), “Monopile Worries Mount: Grouted Joint Doubts Linger”, April 10th 2012.
http://social.windenergyupdate.com/turbine-supply-chain/monopile-worries-mount-grouted-joint-doubts-linger
22. London High Court of Justice; Judgement Before: Mr. Justice Edwards-Stuart. Between : MT Højgaard a/s [Claimant] and
E.ON Climate and Renewables UK Robin Rigg East and West Ltd. [Defendants]. November 2013. Queen’s Bench Division;
Technology and Construction Court. Neutral Citation Number: [2014] EWHC 1088 (TCC); Case No: HT-12-148.
www.bailii.org/ew/cases/EWHC/TCC/2014/1088.html

Contact Details
Dr. C.R. Golightly, BSc, MSc, PhD, MICE, FGS.
Geotechnical and Engineering Geology Consultant
Rue Marc Brison 10G, 1300 Limal, Belgium
Tel. +32 10 41 95 25
Mobile: +44 755 4612888
Email: chris.golightly@hotmail.com
skype: chrisgolightly
Linked In: linkedin.com/pub/5/4b5/469
Twitter: @CRGolightly
Academia.edu: https://independent.academia.edu/ChristopherGolightly
“You Pay for a Site Investigation - Whether You
do One or Not” – Cole et al, 1991.
“Ignore The Geology at Your Peril” – Prof. John
Burland, Imperial College.

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